Typically, in a time-of-flight mass spectrometer (TOF-MS), the time required for an ion to fly a certain distance is measured so as to calculate the mass of the ion from the time of flight, based on the fact that an ion accelerated by a certain amount of energy has a flight speed corresponding to its mass. Accordingly, increasing the flight distance is particularly effective for improving the mass resolving power. However, increasing the flight distance along a straight line is impractical because it inevitably leads to an increase in the size of the apparatus. To overcome this limitation, a variety of optical systems with different configurations for forming a flight space have been conventionally devised.
One example of such ion optical systems is a multi-turn system in which a closed loop orbit having a substantially elliptical shape or substantially figure “8”-shape is formed by using a plurality of sector-shaped electric fields (for example, refer to Patent Document 1 or other documents). In this system, the flight distance of ions can be increased by making them repeatedly fly along the loop orbit many times.
In the multi-turn time-of-flight mass spectrometer, it is necessary to prevent the deterioration of the sensitivity or resolving power due to a temporal or spatial dispersion of ions having the same mass (to be exact, mass-to-charge ratio m/z) during their flight through the loop orbit. To meet this demand, the ion optical system forming the loop orbit (an ion optical system forming the loop orbit is hereinafter simply referred to as an ion optical system) must not only satisfy the condition that its orbit is geometrically closed; it is also essential to prevent the peak from broadening on the time-of-flight axis after the flight through the loop orbit as well as the ion beam from dispersing after the flight through the loop orbit.
To meet such requirements, for example, in the multi-turn time-of-flight mass spectrometer disclosed in Patent Document 1, it is necessary to satisfy the temporal focusing condition that the time of flight of ions after the flight through the loop orbit should be independent of the initial position, initial angle and initial energy of the ions at the beginning of the flight. This requirement limits the shape and/or arrangement the sector-shaped electric fields forming the ion optical system. Such a system is not always easy to design.
The mass resolving power can be enhanced by increasing the number of turns through the loop orbit. However, when ions having different masses are mixed, an ion having a smaller mass and flying faster catches and overtakes another ion having a larger mass and flying more slowly, which makes it difficult to distinguish these ions. Accordingly, in order to enhance the mass resolving power, it is desirable to maximally elongate the one-turn length of the loop orbit so that no catching or overtaking of ions having different masses will occur. The elongation of the one-turn distance requires using a larger number of sector-shaped electric fields to form the ion optical system, increasing their radius of curvature, or elongating the length of the free-flight spaces. In the end, this also requires an enlargement of the installation area of the ion optical system.
One method for preventing the catching and overtaking of the ions on the loop orbit simultaneously with saving the installation area is to form a helical flight orbit. For example, in the apparatuses described in Non-Patent Documents 1 through 3, a loop orbit which is stable on a plane and capable of focusing ions having various kinds of spreads (or dispersion) is slightly shifted in the direction perpendicular to the plane to form a helical orbit. With this configuration, the focusing condition (particularly, the temporal focusing condition) of the ions is satisfied as long as the loop orbit lies on a plane. However, this does not absolutely guarantee that the focusing condition of the ions for the entire helical orbit will also be satisfied. Therefore, for example, it is possible that the sensitivity is deteriorated due to the dispersion of a portion of the ions or that the achieved mass accuracy or mass resolving power is lower than expected. Particularly, these problems are likely to occur when the number of turns is increased to elongate the flight distance.    Patent Document 1: JP-A H11-297267    Non-Patent Document 1: H. Matsuda, “Improvement of a TOF Mass Spectrometer with Helical Ion Trajectory,” Journal of the Mass Spectrometry Society of Japan, 49, p. 227 (2001)    Non-Patent Document 2: H. Matsuda, “Spiral Orbit Time of Flight Mass Spectrometer,” Journal of the Mass Spectrometry Society of Japan, 48, p. 303 (2000)    Non-Patent Document 3: T. Satoh et al., “A New Spiral Time-of-Flight Mass Spectrometer for High Mass Analysis,” Journal of the Mass Spectrometry Society of Japan, 54, p. 11 (2006)